A differential pressure sensor chip including a pressure sensor chip using a sensor diaphragm that outputs a signal depending on a difference between pressures received by a first surface and a second surface of the sensor diaphragm, has been employed so far as a differential pressure sensor for industrial purposes.
In the above differential pressure sensor, measuring pressures applied to pressure receiving diaphragms on a higher pressure side and a lower pressure side are introduced to one surface and the other surface of the sensor diaphragm through enclosed liquids each serving as a pressure transmission medium, and a strain of the sensor diaphragm is detected as chance in a resistance value of a resistance strain gauge, for example. The change in the resistance value is converted to and taken out as an electrical signal.
The differential pressure sensor of the above-mentioned type is used, for example, to measure the height of a liquid level by detecting a difference in pressure between two upper and lower positions inside a sealed tank storing a fluid to be measured, such as a high-temperature tube reactor in an oil refinery plant.
FIG. 10 illustrates a structure of a related-art differential pressure sensor in a schematic form. In this differential pressure sensor 100, a pressure sensor chip 1 including a sensor diaphragm (not illustrated) is assembled in a meter body 2. The sensor diaphragm in the pressure sensor chip 1 is made of, e.g., silicon or glass. A resistance strain gauge is formed on a surface of the diaphragm in the shape of a thin plate. The meter body 2 is constituted by a main body portion 3 made of metal, and a sensor portion 4. Barrier diaphragms (pressure receiving diaphragms) 5a and 5b constituting a pair of pressure receiving portions are disposed respectively in lateral surfaces of the main body portion 3. The pressure sensor chip 1 is assembled in the sensor portion 4.
In the meter body 2, the pressure sensor chip 1 assembled in the sensor portion 4 and the barrier diaphragms 5a and 5b disposed in the main body portion 3 are held in communication with each other through pressure buffer chambers 7a and 7b, respectively, which are partitioned by a center diaphragm 6 having a large diameter. Pressure transmission media 9a and 9b each being silicone oil, for example, are enclosed in communication paths 8a and 8b that interconnect the pressure sensor chip 1 and the barrier diaphragms 5a and 5b, respectively.
The reason why the pressure medium, e.g., silicone oil, is required resides in preventing foreign matters inside a medium as a measurement target from adhering to the sensor diaphragm, and in separating the pressure receiving diaphragm having corrosion resistance and the sensor diaphragm having stress (pressure) sensitivity from each other, thus preventing corrosion of the sensor diaphragm.
In the above differential pressure sensor 100, as seen from FIG. 11A schematically illustrating an operation mode in a steady state, first fluid pressure (first measuring pressure) Pa from a process is applied to the barrier diaphragm 5a, and second fluid pressure (second measuring pressure) Pb from a process is applied to the barrier diaphragm 5b. Accordingly, the barrier diaphragms 5a and 5b are displaced, and the applied pressures Pa and Pb are introduced respectively to one surface and the other surface of the sensor diaphragm in the pressure sensor chip 1 through the pressure buffer chambers 7a and 7b, which are partitioned by the center diaphragm 6, with the aid of the pressure transmission media 9a and 9b. As a result, the sensor diaphragm in the pressure sensor chip 1 exhibits a displacement corresponding to a differential pressure ΔP between the pressures Pa and Pb introduced to the sensor diaphragm.
On the other hand, when overpressure Pover is applied to the barrier diaphragm 5a, for example, the barrier diaphragm 5a is greatly displaced as illustrated in FIG. 11B, and the center diaphragm 6 is responsively displaced so as to absorb the overpressure Pover. When the displacement of the barrier diaphragm 5a is restricted upon contacting a bottom surface (overpressure protective surface) of a recess 10a formed in the meter body 2, further transmission of the differential pressure ΔP to the sensor diaphragm through the barrier diaphragm 5a is prevented. When the overpressure Pover is applied to the barrier diaphragm 5b, the barrier diaphragm 5b is brought into contact with a bottom surface (overpressure protective surface) of a recess 10b formed in the meter body 2 and the displacement of the barrier diaphragm 5b is restricted, whereby further transmission of the differential pressure ΔP to the sensor diaphragm through the barrier diaphragm 5b is prevented, as in the case where the overpressure Pover is applied to the barrier diaphragm 5a. As a result, damage of the pressure sensor chip 1 due to application of the overpressure Pover, i.e., damage of the sensor diaphragm in the pressure sensor chip 1, is avoided.
In the above differential pressure sensor 100, because the pressure sensor chip 1 is incorporated inside the meter body 2, the pressure sensor chip 1 can be protected against external corrosive environments, such as a process fluid. However, it is unavoidable for the differential pressure sensor 100 to have a large size in its external shape because of the structure of protecting the pressure sensor chip 1 against the overpressure Pover with the provision of the recesses 10a and 10b that function to restrict the displacements of the center diaphragm 6 and the barrier diaphragms 5a and 5b. 
In consideration of the above point, there is provided a structure in which a first stopper member and a second stopper member are disposed in a pressure sensor chip, and in which recesses of the first stopper member and the second stopper member are positioned to face one surface and the other surface of the sensor diaphragm, respectively, to avoid excessive displacements of the sensor diaphragm when overpressure is applied, thereby preventing damage and breakage of the sensor diaphragm (see, e.g., Patent Literature (PTL) 1).
FIG. 12 is a schematic representation of a pressure sensor chip employing the structure disclosed in PTL 1. In FIG. 12, reference sign 51-1 denotes a sensor diaphragm, 51-2 and 51-3 denote respectively first and second stopper members bonded to each other with the sensor diaphragm 51-1 interposed therebetween, and 51-4 and 51-5 denote first and second bases bonded to the stopper members 51-2 and 51-3, respectively. The stopper members 51-2 and 51-3 and the bases 51-4 and 51-5 are each made of, e.g., silicon or glass.
In the disclosed pressure sensor chip 51, recesses 51-2a and 51-3a are formed in the stopper members 51-2 and 51-3, respectively. The recess 51-2a in the stopper member 51-2 is positioned to face one surface of the sensor diaphragm 51-1, and the recess 51-3a in the stopper member 51-3 is positioned to face the other surface of the sensor diaphragm 51-1. The recesses 51-2a and 51-3a have curved surfaces (aspherical surfaces) corresponding to displacements of the sensor diaphragm 51, and pressure introduction holes (pressure guide holes) 51-2b and 51-3b are formed at the bottoms of the recesses 51-2a and 51-3a, respectively. Moreover, pressure introduction holes (pressure guide holes) 51-4a and 51-5a are formed in the bases 51-4 and 51-5 at positions corresponding to the pressure introduction holes 51-2b and 51-3b in the stopper members 51-2 and 51-3, respectively.
In the case of employing the pressure sensor chip 51, when the sensor diaphragm 51-1 is displaced upon application of overpressure to the one surface of the sensor diaphragm 51-1, the displaced surface of the sensor diaphragm 51-1 is entirely received by the curved surface of the recess 51-3a in the stopper member 51-3. Furthermore, when the sensor diaphragm 51-1 is displaced upon application of overpressure to the other surface of the sensor diaphragm 51-1, the displaced surface of the sensor diaphragm 51-1 is entirely received by the curved surface of the recess 51-2a in the stopper member 51-2.
As a result, an excessive displacement caused upon application of overpressure to the sensor diaphragm 51-1 is prevented, and stresses are avoided from concentrating at a peripheral edge portion of the sensor diaphragm 51-1. It is hence possible to effectively prevent unintended breakage of the sensor diaphragm 51-1, which would be caused by the application of overpressure, and to increase overpressure protection operating pressure (i.e., withstanding pressure). Moreover, in the structure illustrated in FIG. 10, the size of the meter body 2 can be reduced with the modification of eliminating the center diaphragm 6 and the pressure buffer chambers 7a and 7b, and of directly introducing the measuring pressures Pa and Pb to the sensor diaphragm 51-1 from the barrier diaphragms 5a and 5b, respectively.